1. Field of the Invention
The invention pertains to an optical waveguide component comprising a substrate, a core-matching refractive index lower cladding layer, a core layer, a core-matching refractive index upper cladding layer, and a low refractive index top cladding.
2. Description of the Related Art
Such a component is known from, e.g., International patent application WO 97/01782. This publication concerns optical components having an at least penta-layered polymer structure on a substrate comprising: a) a low refractive index bottom cladding layer, b) a core-matching refractive index lower cladding layer, c) a core layer, d) a core-matching refractive index upper cladding layer, and e) a low refractive index top cladding layer.
With this specific layer structure optimum confinement in the direction of the thickness of the stack of layers (also denoted as transverse direction} can be obtained, which results in less loss of light and an improved switching efficiency. However, present and future applications of optical devices require crosstalk to be as low as possible.
Accordingly, it is an object of the present invention to further reduce crosstalk. This is achieved, in the components described in the first paragraph, by leaving out the low refractive index bottom cladding layer and employing a single core-matching refractive index lower cladding layer which has a thickness sufficient to avoid substantial capture and/or absorption by the substrate of a guided mode in the core layer, whereas slab modes, quasi-guided modes and/scattered light leak to the substrate.
It was found that, in the penta-layered components, slab modes, quasi-guided modes and/or scattered light (sometimes also referred to as radiation modes and stray light respectively) become trapped between the lower and upper cladding layers, with the core layer and the core-matching refractive index cladding layers serving as a (composite) core. By leaving out the bottom cladding layer, making sure that the slab modes, quasi-guided modes and/scattered light leak to the substrate, and selecting the thickness of the core-matching refractive index tower cladding layer such that substantial absorption of a guided mode by the substrate is avoided, crosstalk and other detrimental phenomena are significantly reduced. Once captured or absorbed by the substrate, the said modes cannot have any interaction with the guide mode(s) in the core layer and a decrease of the optical performance is avoided. E.g., for an 1xc3x972 optical switch, avoiding interaction between a guided mode in the core layer and slab modes, quasi-guided modes and/scattered light will result in a considerably improved isolation (defined as the ratio of the optical power in an output in the on-state and the optical power in an output in the off-state).
Capture and/or absorption of the guided mode should preferably be smaller than 0.01 dB/cm, more preferably smaller than 0.001 dB/cm.
Leaking of slab modes, quasi-guided modes and/or scattered light to the substrate can be achieved by using a substrate that has a refractive index higher than that of the core-matching refractive index lower cladding layer and/or that functions as an absorber of the said undesirable modes. Any material that absorbs and dissipates light in the optical frequencies used in the structure in question will do. Examples of suitable materials are metals such as titanium, silver, gold, or nickel or non-transparent dielectric polymers containing a dye. It is noted that the substrate may comprise one or more (usually very thin) top layers or coatings, e.g., to promote adhesion to the core-matching refractive index lower cladding layer.
The components according to the invention allow high switching speeds and high confinement of a guided mode, require less power for switching and, due to the capture or absorption by the substrate of slab modes, quasi-guided modes and/or scattered light, exhibit less crosstalk.
It is noted that EP 642 052 discloses a polymeric thermo-optical device comprising a polymeric core layer sandwiched between two cladding layers having a refractive index lower than that of the guiding layer. A heating element is placed against one of the cladding layers and this layer has a lower refractive index than the other cladding layer. In a particularly preferred embodiment, the lower cladding layer is made up of two sublayers to provide optical isolation from the substrate. Thus, the gist of EP 642 052 runs counter to that of the present invention.